Combustion engines, such as those used in power plants and motor vehicles, emit substances that are harmful to the environment, including nitrogen oxide. Nitrogen oxides such as nitric oxide (NO) and nitrogen dioxide (NO2) are extremely toxic. Nitric oxide and nitrogen dioxide are often present when nitrous oxide (N2O) is produced during combustion.
When NOx and volatile organic compounds react in the presence of sunlight they form photochemical smog. This smog is a significant form of air pollution, especially in the summertime. Children, people with lung diseases, such as asthma, and people who work or exercise outside are susceptible to adverse effects of smog such as damage to lung tissue and reduction in lung function. Also, NOx forms nitric acid when dissolved in atmospheric moisture, forming a component of acid rain. As is well known, acid rain causes negative visual and physical affects to buildings and other structures. Furthermore, any unnatural level of nitric acid in the environment due to acid rain is undesirable.
There are conventional ways to remove NOx such as by filtering, absorption, and/or catalytic reactions. In order to remove NOx from emissions of internal combustion engines, one of the conventional methods is selective catalytic reduction (SCR), which is conventionally used for diesel engines, for example.
Conventional applications of SCR to reduce NOx emissions for motor vehicles include injection of ammonia supplied from a tank of liquid ammonia or urea (a major component of which is ammonia) into the emissions stream of the diesel engines. However, the liquid ammonia supply has to be maintained and replenished. Planning and performing periodic maintenance to check and replenish urea supplies is costly and, hence, can be dismissed, despite the negative environmental effects of NOx emissions